In optical data recording, an optical source, typically a laser or laser diode, generates an incident write signal in the form of a radiation beam. The beam is applied to an optical medium to record data thereon as optically-detectable marks. The quality of recorded data in many optical recording systems is generally very sensitive to laser write power.
A commonly used technique for determining an initial optimum laser write power involves allocating a section of the recording media for power calibration measurements. Several recordings are made in the calibration section using a range of laser powers. The calibration recordings are read back, and the write power level which produced the best recording quality is selected as optimum. A measure of recorded data quality commonly used in compact disk (CD) recording is referred to as Beta, or simply .beta.. .beta. is defined in the Orange Book attachment B3.3. "Orange Book" is a licensed specification published by Philips Corporation and Sony Corporation which defines key properties of recordable compact disk (CD-R) media and recording procedures. .beta. measures recorded mark length error by comparing positive and negative peaks of an AC-coupled readback waveform. Because the CD format uses a DC free data encoding scheme, when the marks have the proper length, .beta. equals its target value (usually near zero). When the mark length increases, due to, for example, a recording power that is too high, .beta. is more positive than its target value. Likewise, when the mark length decreases, .beta. is more negative than its target value. The Optimum Recording Power (ORP) is the power which results in the target .beta. value and is then used to record actual data in the program or user area of the media.
In most recording systems, however, recording power may need to be adjusted during recording because ORP varies as various system parameters fluctuate, for example, with media sensitivity, defocus, tilt, substrate thickness, birefringence, scratches, and contamination on the laser-incident substrate surface. One technique for continuously maintaining the ORP involves monitoring a reflection of the write signal from the medium, known as the Mark Formation (MF) signal, while data is being recorded. Systems which monitor the MF signal are generally referred to as Direct Read During Write (DRDW) systems. The MF signal is also used to avoid the need to subsequently read the data after recording by analyzing the MF signal to determine whether or not a mark has been properly formed, or, in other words, whether the data has been properly recorded on the medium.
FIG. 1 shows an MF signal for a typical prior art "burn bright" optical recording medium, in which a mark is an area of increased reflectivity, and a front facet (FF) signal from one laser write pulse of duration 8 Tclock. The FF signal is produced by a detector placed in front of the laser, also known as a front facet monitor (FFM), and can be used to monitor laser power. Tclock is the channel clock period. For CD-R format, for example, the minimum mark length is 3 channel clock periods, or 3 Tclock.
As shown in FIG. 1, the MF signal varies with time, t, and has a rate of change resulting from a change in reflectivity of the optical medium as the mark is formed thereon. For t&lt;0, the MF signal is the reflected intensity of the unmarked medium at the read laser power. Increasing the laser power to a writing level at t=0 in order to form a mark results in an increase in the MF signal. During the write pulse, the MF signal continues to gradually increase. The maximum steady state signal attained is referred to as a "plateau." Subsequent to the high power laser write pulse, at t=8 Tclock, the laser reverts to read power. The MF signal decreases to above that of the unrecorded medium because of the finite extent of the optical spot, which samples both the unwritten and written portions of the medium. At t&gt;8 Tclock, the MF signal gradually settles back to its original value at t&lt;0. For burn bright media, the difference between the initial level of the MF signal at the start of the laser write pulse and the final plateau level at the end of the laser write pulse is relatively low. Because of its small amplitude, the MF signal is easily distorted by fluctuations in the laser write power level during mark formation.
Certain characteristics of the MF signal, including, for example, the voltage levels before and during the steady state plateau portion of the MF signal during a write pulse, as well as any estimates or transformations thereof, can be used to determine whether the laser write power level needs to be adjusted. These MF signal characteristics are generally referred to as Mark Formation Effectiveness (MFE) signals. During calibration, an optimum value of an MFE signal can be determined and stored. A servo loop can then be used during actual data recording to adjust the laser write power when the MFE signal deviates from the optimum, or previously stored reference, MFE signal value. The optimum MFE signal, for example, might be the value during recording which results in the target .beta. value. A technique to servo the laser write power on optical recording media during recording is disclosed, for example, in commonly assigned U.S. Pat. No. 5,436,880, entitled "Laser Power Control In An Optical Recording System Using Partial Correction of Reflected Signal Error," and in commonly assigned U.S. Pat. No. 5,446,716, entitled "Laser Power Control In An Optical Recording System to Compensate For Multiple System Degradations," the disclosures of which are incorporated herein by reference.
Apparatus for generating an MFE signal to verify data as it is recording on an optical medium is disclosed in commonly assigned U.S. Pat. No. 5,495,466, entitled "Write Verification In An Optical Recording System By Sensing Mark Formation While Writing DRDW with Recordable Compact Disk," the disclosure of which is incorporated by reference. The MFE signal generated in U.S. Pat. No. 5,495,466 estimates a normalized rate of change of the reflected write pulse as a mark is being formed to provide an indication of the quality of mark formation on the optical medium. A technique to generate an MFE signal to control laser write power during recording is also disclosed in U.S. Pat. No. 5,216,660, entitled "Method of Optimally Controlling the Power of a Recording Laser Beam," assigned to Sony Corporation.
These techniques are especially useful for "burn dark" optical recording media, such as recordable compact disks (CD-R's), in which a mark is an area of reduced reflectivity. The media reflectivity declines from a high reflectivity value to a low reflectivity value during the course of a write pulse. In other words, with burn dark media, the difference between the initial peak of the MF signal at the start of the laser write pulse and the final plateau level during mark formation is high. Thus, MF signals for typical burn dark media may be less affected by fluctuations in laser write power during mark formation. However, MF signals for burn bright media can be affected to a much larger degree by fluctuations of laser power during mark formation. As a result, the MFE signals generated using prior art techniques are much more susceptible to error.